Power generation unit, secondary cell, and electronic apparatus

ABSTRACT

A power generation unit includes a deforming member adapted to repeatedly deform a piezoelectric element, a pair of electrodes provided to the piezoelectric element, an inductor disposed between the pair of electrodes, and constituting a resonant circuit together with a capacitive component of the piezoelectric element, a first switch connected in series to the inductor, a member adapted to detect a timing at which a deformation direction of the deforming member is switched, a full bridge rectifier adapted to rectify a current output from the pair of electrodes, a capacitor connected to the full bridge rectifier, and adapted to store a current supplied from the full bridge rectifier, a second switch connected between either one of the pair of electrodes and the capacitor, and a control circuit adapted to operate the first switch and the second switch.

BACKGROUND

1. Technical Field

The present invention relates to a power-generating device, a secondarycell, and an electronic apparatus.

2. Related Art

When a piezoelectric material such as lead zirconium titanate (PZT),quartz crystal (SiO₂), or zinc oxide (ZnO) is deformed in response to anexternal force, electrical polarization is induced inside the material,and positive and negative charges appear on the surface. Such aphenomenon is called a piezoelectric effect. There has been proposed anelectrical power generation method of vibrating a cantilever to therebymake a weight repeatedly act on the piezoelectric material, and thustaking out the charge generated on the surface of the piezoelectricmaterial as a current using such a characteristic the piezoelectricmaterial has.

For example, by vibrating a metal cantilever having a mass disposed atthe tip and a thin plate made of the piezoelectric material bondedthereto, and taking out the positive and negative charges generated onthe piezoelectric material due to the vibration, an alternating currentis generated. Then the alternating current is rectified by diodes, andthen stored in a capacitor, and then taken out as electricity. Such atechnology has been proposed in, for example, JP-A-7-107752. Further,there has been also proposed a technology of arranging that a junctionis closed only in the period during which the positive charges aregenerated in a piezoelectric element to thereby make it possible toobtain a direct current without causing a voltage loss in the diodes(see, e.g., JP-A-2005-312269). By using these technologies, downsizingof the power generation unit can be achieved. Therefore, an applicationsuch as incorporation of the power generation unit to a small-sizedelectronic component instead of a battery is expected.

However, in the proposed power generation unit according to the relatedart, there arises a problem that the obtainable voltage is limited up tothe voltage generated by the electrical polarization of thepiezoelectric material. Therefore, in most cases, an additional step-upcircuit is required, and there arises a problem that it is difficult tosufficiently downsize the power generation unit. Further, an electricalpower is usually required to drive the step-up circuit, and there arisesa problem that the step-up operation becomes difficult if the electricalenergy in the capacitor is reduced. Although it is also possible todispose a full bridge rectifier or a voltage doubler rectifier inparallel to the step-up circuit in order for solving this problem, therearises a problem that the power generation unit grows in size.

SUMMARY

An advantage of some aspects of the invention is to solve at least apart of the problems described above, and the invention can beimplemented as the following forms or application examples.

Application Example 1

This application example is directed to a power generation unitincluding: a piezoelectric element including a piezoelectric material, adeforming member adapted to repeatedly deform the piezoelectric element,a pair of electrodes provided to the piezoelectric element, an inductorelectrically connected to the piezoelectric element, a first switchdisposed between the piezoelectric element and the inductor, a memberadapted to detect a timing at which a deformation direction of thedeforming member is switched, a full bridge rectifier adapted to rectifya current output from the pair of electrodes, a capacitor electricallyconnected to the full bridge rectifier, and adapted to store a currentsupplied from the full bridge rectifier, a second switch disposedbetween either one of the pair of electrodes and the capacitor, and acontrol circuit adapted to operate the first switch and the secondswitch.

According to this application example, by repeatedly deforming thepiezoelectric element while switching the deformation direction inaccordance with the external force, the positive and negative chargesare generated in the piezoelectric element due to the piezoelectriceffect. Since the piezoelectric element can be assumed to be a capacitorfrom the viewpoint of the electrical circuit, when shorting the firstswitch to thereby connect the piezoelectric element to the inductor, aresonant circuit is formed of the piezoelectric element and the inductorconnected to each other. Then, the charge generated in the piezoelectricelement flows into the inductor. Then, since the piezoelectric elementand the inductor constitutes the resonant circuit, the current havingflown into the inductor overshoots, and then flows into thepiezoelectric element through the terminal on the opposite side. Thus,it is possible to reverse the arrangement of the positive and negativecharges having been generated in the piezoelectric element beforeconnecting the inductor thereto.

Then, when deforming the piezoelectric element in the opposite directionin turn in this state, it results that the charge generated due to thepiezoelectric effect is accumulated in addition to the charge reversedand then accumulated. As a result, it becomes possible to accumulate thecharge generated by repeatedly deforming the piezoelectric element inthe piezoelectric element. Further, since the voltage between theterminals increases in accordance with the charge accumulated in thepiezoelectric element, it is possible to generate a voltage higher thanthe voltage generated by the electrical polarization of thepiezoelectric material without additionally preparing a step-up circuit.As a result, a small-sized and efficient power generation unit can beobtained.

Here, in order to perform the step-up operation described above, itbecomes necessary to actively control the first switch forconnecting/disconnecting the piezoelectric element and the inductor.Specifically, once the voltage applied to the control circuit drops to alevel lower than the lower limit voltage necessary to drive the firstswitch, it becomes unachievable to actively control the first switch,and thus, it results that the current is rectified by the full bridgerectifier and is then stored without performing the step-up operationdescribed above.

Since the voltage, which can be stored in the capacitor, is low in thecase of rectifying the current with the full bridge rectifier and thenperforming the storage, it is difficult to supply the control circuitwith the voltage equal to or higher than the lower limit voltagenecessary to drive the first switch. Therefore, in such a case, thecontrol circuit sets the second switch to a connected state to therebyswitch the circuit to the voltage doubler rectifier using the capacitivecomponent of the piezoelectric element. Thus, since it is possible tostore the voltage approximately twice as high as the voltage by the fullbridge rectifier, the voltage applied to the control circuit exceeds thelower limit voltage necessary to drive the first switch. Therefore, theself-recovery to the step-up operation described above can be achieved.Further, since the voltage doubler rectification is performed using thecapacitive component of the piezoelectric element, no additionalcomponents are required, and a small-sized and low-cost power generationunit can be provided.

Application Example 2

This application example is directed to the power generation unitaccording to the application example described above, wherein when avoltage stored in the capacitor reaches a voltage with which the controlcircuit can drive the second switch, the control circuit sets the firstswitch to an open state, and the second switch to a shorted state.

According to this application example, by setting the first switch tothe open state and the second switch to the shorted state when thevoltage stored in the capacitor has reached the voltage with which thecontrol circuit can drive the second switch, it becomes possible tostore the voltage approximately twice as high as the voltage by the fullbridge rectifier in the capacitor. Therefore, the voltage applied to thecontrol circuit exceeds the lower limit voltage necessary to drive thefirst switch, and the self-recovery to the step-up operation describedabove can be achieved. Further, since the voltage doubler rectificationis performed using the capacitive component of the piezoelectricelement, no additional components are required, and a small-sized andlow-cost power generation unit can be provided.

Application Example 3

This application example is directed to the power generation unitaccording to the application example described above, wherein when avoltage charged in the capacitor reaches a voltage with which thecontrol circuit can drive the first switch and the second switch, thecontrol circuit sets the second switch to an open state, and performs acontrol of setting the first switch to a shorted state at the timing atwhich the deformation direction of the deforming member is switched, andthen setting the first switch to the open state after a timecorresponding to a half cycle of a resonance period of a resonantcircuit composed mainly of the inductor and the capacitor has elapsed.

According to this application example, when performing the step-upoperation by operating the first switch, by setting the second switch tothe open state to thereby make the circuit function as the full bridgerectifier, it is possible to efficiently perform the step-up operation.Further, since the larger the amount of deformation of the piezoelectricelement is, the larger the amount of the charge generated is, by settingthe first switch to the shorted state when the deformation direction isswitched, it is possible to reverse the positive and negative charges inthe piezoelectric element when the amount of the charge stored in thepiezoelectric element is the maximum. The time during which the firstswitch is shorted corresponds to the time necessary for the charges inthe piezoelectric element to be reversed, and by shorting the firstswitch for the period of time corresponding to a half of the resonanceperiod of the resonant circuit formed of the piezoelectric element andthe inductor, the step-up operation can be performed with the highestefficiency.

Application Example 4

This application example is directed to the power generation unitaccording to the application example described above, which furtherincludes a charging state detection section adapted to detect a chargingstate of the capacitor, when the charging state detection sectiondetects a state in which charging of the capacitor stops, the controlcircuit performs control of setting the second switch to a shortedstate, and setting the first switch to an open state.

According to this application example, when performing the step-upoperation, if the charging state detection section fails to detect thecharging, the step-up operation is stopped, and the second switch is setto the shorted state. The state in which the charging state detectionsection fails to detect the charging corresponds to the state in whichno current is flowing from the piezoelectric element to the capacitor,and if the step-up operation is performed in this state, the step-upoperation has no contribution to the electrical power generation. Inother words, the electrical power necessary to perform the step-upoperation is wasted. Therefore, by stopping the step-up operation if thecharging state detection section fails to detect the current, it can beprevented to waste the electrical power. Further, if the step-upoperation is stopped, since only the full bridge rectifier can operate,the generation voltage of the piezoelectric element is dropped to alevel lower than in the case of performing the step-up operation, and itbecomes difficult to make the current flow from the piezoelectricelement to the capacitor. However, by shorting the second switch tothereby switch the circuit to the voltage doubler rectifier, since theoutput voltage of the piezoelectric element increases to a level higherthan in the full bridge rectifier, it becomes easy to supply thecapacitor with the current. According to the above configuration, apower generation unit with a high efficiency can be provided.

Application Example 5

This application example is directed to a secondary cell including thepower generation unit according to any one of the application examplesdescribed above.

According to this application example, the voltage doubler rectificationis performed even in the state in which the output voltage of thesecondary cell is dropped and the control of the first switch isunachievable. Further, by promptly making a transition to the step-upoperation with a high electrical power generation efficiency afterobtaining the voltage equal to or higher than the lower limit voltage,it becomes possible to provide a secondary cell having an electricalpower generation efficiency higher than that of the ordinary full-waverectification, and capable of achieving the self-recovery.

Application Example 6

This application example is directed to an electronic apparatusincluding the power generation unit according to any one of theapplication examples described above.

According to this application example, since the power generation unithaving an electrical power generation efficiency higher than that of theordinary full-wave rectification, and capable of achieving theself-recovery is provided, it becomes possible to provide an electronicapparatus with a high efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a schematic diagram showing a structure of a power generationunit according to an embodiment of the invention.

FIGS. 2A and 2B are circuit diagrams of the power generation unitaccording to the embodiment.

FIGS. 3A through 3D are graphs showing an operation of the powergeneration unit according to the embodiment in a steady state.

FIG. 4A is a circuit diagram showing a current path when a second switchaccording to the embodiment is shorted, and FIG. 4B is an equivalentcircuit diagram extracting the part related to the path through whichthe current shown in FIG. 4A flows.

FIG. 5 is a flowchart of the present embodiment for determining whichone of the operation in the steady state and the operation for obtaininga starting voltage is selected.

FIG. 6 is a circuit diagram of a secondary cell according to theembodiment.

FIG. 7 is schematic diagram showing a schematic structure of a pedometer(a manpo-kei, which is a registered trademark) as an electronicapparatus according to the embodiment.

FIG. 8 is a circuit diagram for explaining a first modified example.

FIGS. 9A and 9B are circuit diagrams for explaining a second modifiedexample.

FIG. 10 is a circuit diagram for explaining a third modified example.

FIG. 11 is a circuit diagram for explaining a fourth modified example.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT Embodiment

Electrical Power-Generating Device having Configuration of The Invention

Hereinafter, an embodiment of the invention will be described withreference to the accompanying drawings. FIG. 1 is a schematic diagramshowing a structure of a power generation unit according to the presentembodiment. The mechanical structure of an electrical power-generatingsection 125 provided to a power generation unit 100 described in thepresent embodiment is a cantilever structure having a beam 104, which isa deforming member, and has a mass 106 disposed on the tip thereof,fixed to a base 102 on a base end side thereof. Further, a piezoelectricelement 108 formed of a piezoelectric material such as lead zirconiumtitanate (PZT) is fixedly supported by a surface of the beam 104 as asupport body, and a first electrode 109 a and a second electrode 109 bas a pair of electrodes using a conductive body such as a metal thinfilm are disposed respectively on the both surfaces of the piezoelectricelement 108. It should be noted that although in the example shown inFIG. 1, the piezoelectric element 108 is disposed on the upper surfaceside of the beam 104, it is also possible to dispose the piezoelectricelement 108 on the lower side of the beam 104, or it is also possible todispose the piezoelectric elements 108 on both of the upper surface sideand the lower surface side of the beam 104. It should be noted that itis assumed that the term “upper” denotes the direction (the positivedirection of “u” in the drawing) in which the first electrode 109 a isviewed from the piezoelectric element 108, and the term “lower” denotesthe opposite direction to the “upper” direction.

Since the beam 104 is fixed to the base 102 at the base end sidethereof, and has the mass 106 disposed on the tip side thereof, whenvibration or the like is applied to the beam 104, the tip of the beam104 vibrates with a large amplitude as indicated by an outlined arrow inthe drawing. As a result, the piezoelectric element 108 attached to thesurface of the beam 104 is provided with a cyclic deformation due to theexternal force, and it results that compressive force and tensile forceact alternately thereon. Then, the piezoelectric element 108 generatespositive and negative charges due to the piezoelectric effect, and thecharges appear on the first electrode 109 a and the second electrode 109b, and are taken out as a current.

FIGS. 2A and 2B are circuit diagrams of the power generation unit 100according to the present embodiment. The piezoelectric element 108 canelectrically be expressed as a current source I0 and a capacitor C0 forstoring a charge. An inductor L is connected in parallel to thepiezoelectric element 108 to thereby form an electrical resonant circuittogether with the capacitance component C0 of the piezoelectric element108. Further, a first switch SW1 for shorting/opening the resonantcircuit is connected in series to the inductor L. Although a secondswitch SW2 is connected to the second electrode 109 b and a capacitor inFIG. 2A, it is also possible to connect the second switch SW2 to thefirst electrode 109 a as shown in FIG. 2B. The shorting/opening of thefirst switch SW1 and the shorting/opening of the second switch SW2 arecontrolled by a control circuit 110. Further, the first electrode 109 aand the second electrode 109 b provided to the piezoelectric element 108are connected to a full bridge rectifier 120 composed of four diodes D1through D4. Although the details will be described later, a member 140for detecting the timing at which the deformation direction is switchedis provided for determining the timing of shorting the resonant circuit.Although in the present embodiment the timing at which the deformationdirection is switched is determined based on the value of the currentflowing from the piezoelectric element to the full bridge rectifier, itis also possible to determine the timing using an output voltage of adisplacement sensor or the piezoelectric element besides the above.Here, junction diodes are used as the diodes D1 through D4. The diodesD1 through D4 function as the full bridge rectifier 120 when the secondswitch SW2 is set to the open state. Further, the diodes D1 through D4function as a rectifier device for converting the alternating currentinto the direct current together with the capacitor C.

Further, the diodes D1 through D4 function as a voltage doublerrectifier 120 a in the case of setting the second switch SW2 to theshort state. The positive and negative charges generated by thepiezoelectric element 108 are taken out by the first electrode 109 a andthe second electrode 109 b, and form an alternating current. Then, thealternating current is converted into a pulsating current by the fullbridge rectifier 120 or the voltage doubler rectifier 120 a providedwith the diodes D1 through D4. Then, the pulsating current is stored inthe capacitor C.

Operation in Steady State

FIGS. 3A through 3D are graphs showing an operation of the powergeneration unit 100 according to the present embodiment in a steadystate. Here, it is assumed that the steady state denotes the case inwhich the voltage capable of controlling the operation of the firstswitch SW1 and the second switch SW2 is supplied to the control circuit110.

FIG. 3A shows a movement of the position of the displacement of the tipof the beam 104. The vertical axis represents the displacement u, andthe horizontal axis represents the time t. The unit of the displacementis an arbitrary unit. As shown in FIG. 3A, it is expressed that thedisplacement u of the tip of the beam 104 varies with the vibration ofthe beam 104. It should be noted that the positive displacement urepresents the state (the state in which the upper surface of the beam104 has a concave shape) in which the beam 104 warped upward, and thenegative displacement (−u) represents the state (the state in which thelower surface of the beam 104 has a concave shape) in which the beam 104warped downward. Further, FIG. 3B shows the state of the currentgenerated by the piezoelectric element 108 due to the deformation of thebeam 104 and the electro motive force caused inside the piezoelectricelement 108 as a result thereof. it should be noted that in FIG. 3B, thestate of the charge generated in the piezoelectric element 108 isexpressed as an amount of charge (i.e., a current Ip) generated per unittime. Here, the vertical axis represents the current Ipzt flowingthrough the piezoelectric element 108. Further, the electro motive forcegenerated in the piezoelectric element 108 is shown with the verticalaxis representing the electrical potential difference Vpzt generatedbetween the first electrode 109 a and the second electrode 109 b. In thecase of performing the operation in the steady state, the second switchSW2 is opened.

As shown in FIGS. 3A and 3B, during the period in which the displacementof the beam 104 is increasing, the piezoelectric element 108 generatesthe current of the positive direction (i.e., the current Ip takes apositive value). In accordance therewith, the electrical potentialdifference Vp between the first electrode 109 a and the second electrode109 b increases in the positive direction. If the electrical potentialdifference Vp in the positive direction exceeds the sum of the voltageVC1 between the terminals of the capacitor C and a twofold of theforward voltage drop Vf of the diode constituting the full bridgerectifier 120, namely VC1+2 Vf, the charge generated thereafter can betaken out as a direct current and stored in the capacitor C. Further,during the period in which the displacement of the beam 104 isdecreasing, the piezoelectric element 108 generates a current of thenegative direction (i.e., the current Ip takes a negative value). Inaccordance therewith, the electrical potential difference Vp between thefirst electrode 109 a and the second electrode 109 b increases in thenegative direction. If the electrical potential difference Vp in thenegative direction exceeds the sum of VC1 and 2 Vf of the full bridgerectifier 120, the charge generated can be taken out as a direct currentand stored in the capacitor C. A typical electrical power-generatingmethod is as described hereinabove. Here, a method of performing a moreefficient electrical power generation by controlling the first switchSW1 will be explained.

FIG. 3C is a graph showing the timing of shorting (ON) the first switchSW1, wherein the first switch SW1 is in the ON state only in the periodindicated as “ON.” FIG. 3D shows a voltage waveform obtained in the casein which the first switch SW1 is shorted (ON) at the timing shown inFIG. 3C. The vertical axis represents the electrical potentialdifference Vgen generated between the first electrode 109 a and thesecond electrode 109 b as the generated voltage.

The first switch SW1 is shorted (ON) at the timing (the timing at whichthe displacement of the piezoelectric element 108 takes a local maximumor a local minimum) shown in FIG. 3C. Then, as shown in FIG. 3D, thereoccurs a phenomenon that the voltage waveform between the firstelectrode 109 a and the second electrode 109 b sandwiching thepiezoelectric element 108 varies as if it is shifted at the moment thatthe first switch SW1 is shorted. For example, in the period B indicatedas “B” in FIG. 3D, such a voltage waveform indicated by the thick dottedline as is obtained by shifting the electrical potential difference Vpindicated by the thin dotted line corresponding to the electro motiveforce of the piezoelectric element 108 toward the negative directionappears between the first electrode 109 a and the second electrode 109 bsandwiching the piezoelectric element 108.

Further, in the period C indicated as “C” in FIG. 3D, there appears sucha voltage waveform indicated by the thick dotted line as is obtained byshifting the electrical potential difference Vp corresponding to theelectro motive force of the piezoelectric element 108 toward thepositive direction. Similarly, thereafter, in each of the period D, theperiod E, the period F, and so on, there appears such a voltage waveformindicated by the thick dotted line as is obtained by shifting theelectrical potential difference Vp corresponding to the electro motiveforce of the piezoelectric element 108 toward the positive direction orthe negative direction.

These can be obtained using the resonant phenomenon in the resonantcircuit provided with the inductor L and the capacitive component C0 ofthe piezoelectric element 108. When shorting (ON) the first switch SW1at the timing (when the displacement takes −u in FIG. 3A) when thedisplacement of the piezoelectric element 108 takes a local minimum, thecurrent flowing through the inductor L start flowing gradually againstthe inductance of the inductor L. Then, when the voltage between theboth ends of the capacitive component C0 vanishes, the current flowingthrough the inductor L becomes the maximum. Subsequently, the currentcontinues to flow due to the inductance of the inductor L, and thenvanishes at the state in which the voltage between the both ends of thecapacitive component C0 is reversed. Then, the first switch SW1 isopened (OFF).

After then, it results that the piezoelectric element 108 is warped tothe opposite direction. In other words, the current Ip takes a positivevalue, and charges the capacitive component C0 in the positivedirection. Since the charge stored in the capacitive component C0 isheld in the inverted state in the operation described above, a valuelarger than the value, which can be generated by the piezoelectricelement 108 in the typical operation, can be taken by newly adding thepositive charge.

Then, by performing the same operation when the displacement of thepiezoelectric element 108 takes a local maximum (when the displacementtakes u in FIG. 3A), the negative electrical potential difference Vpwith an absolute value larger than the value, which can be generated inthe typical operation, is generated in turn. In other words, by shortingthe first switch SW1 at the timing when the displacement of thepiezoelectric element 108 takes a local maximum or a local minimumduring the period corresponding to a half cycle of the resonance periodof the resonant circuit composed of the capacitive component C0 and theinductor L, it becomes possible to more efficiently take out theelectrical power from the piezoelectric element 108.

In this case, the charge in the piezoelectric element 108 increasesevery time the piezoelectric element 108 is deformed, unless the chargeis made to flow out of the piezoelectric element 108. Therefore, thevoltage between the first electrode 109 a and the second electrode 109 bsandwiching the piezoelectric element 108 increases.

Here, in the part (the part indicated by hatching in FIG. 3D) where thevoltage exceeds the sum of VC1 and 2 Vf, the charge generated by thepiezoelectric element 108 is stored in the capacitor C. Therefore, thecharge is made to flow from the piezoelectric element 108 to thecapacitor C, and the voltage between the first electrode 109 a and thesecond electrode 109 b is clipped at the voltage (VC1+2 Vf)corresponding to the sum of the inter-terminal voltage of the capacitorC and 2 Vf. As a result, the waveform indicated by the thick solid linein FIG. 3D is obtained as the voltage waveform of the voltage betweenthe first electrode 109 a and the second electrode 109 b.

As is obvious from the comparison between the case of keeping the firstswitch SW1 open shown in FIG. 3B and the case of shorting the firstswitch SW1 at the timing when the deformation direction of the beam 104is switched, in the power generation unit 100 according to the presentembodiment, it becomes possible to efficiently store the charge in thecapacitor C by shorting/opening the first switch SW1 at appropriatetiming.

Further, if the charge is stored in the capacitor C and the voltagebetween the both terminals of the capacitor C increases, the shiftamount of the voltage waveform also increases in accordance therewith.For example, in comparison between the period B (the state in which nocharge is stored in the capacitor C) in FIG. 3D and the period H (thestate in which the charge is stored in the capacitor C) in FIG. 3D, theshift amount of the voltage waveform is larger in the period H.Similarly, in comparison between the period C and the period I in FIG.3D, the shift amount of the voltage waveform is larger in the period Iin which the charge stored in the capacitor C is increased. As a result,in the power generation unit 100 according to the present embodiment, itbecomes possible to store the voltage higher than the electricalpotential difference Vp generated between the first electrode 109 a andthe second electrode 109 b by deforming the piezoelectric element 108.As a result, it become unnecessary to provide an additional step-upcircuit, and thus it becomes possible to obtain a small-sized and highlyefficient power generation unit. Hereinafter, this operation is referredto as a step-up operation.

If the timing at which the control circuit 110 turns ON the switch SWand the timing at which the deformation direction of the beam 104 isswitched do not strictly coincide with each other, by turning the switchSW for a period corresponding to a half of the resonance period of theresonant circuit, which is composed of the inductor L and the capacitivecomponent C0 of the piezoelectric element 108, at a predeterminedperiod, it is possible to raise the voltage Vgen between the bothterminals of the piezoelectric element 108. It should be noted that thedevice in which the timing at which the switch SW is turned ON and thetiming at which the deformation direction of the beam 104 is switchedcoincide with each other has the highest efficiency, and the device inwhich the timing at which the switch SW is turned OFF and the timing atwhich the deformation direction of the beam 104 is switched coincidewith each other has the lowest efficiency. In other words, the closerthe timing at which the switch SW is turned ON and the timing at whichthe deformation direction of the beam 104 is switched are, the higherthe electrical power generation efficiency is.

Operation with No Starting Voltage Applied

Going back to FIGS. 2A and 2B, although in the case in which the voltagesufficient for the control circuit 110 to operate is supplied in theinitial state, it is possible to take out the electrical power providedby the piezoelectric element 108 at a higher efficiency compared to therelated art due to the step-up operation described above, if theoperating voltage (e.g., 3.3V) is not applied to the control circuit 110at the time of startup, the first switch SW1 fails to be shorted/opened,and therefore, the step-up operation fails to be performed. Therefore,it becomes necessary to once store the electrical power for starting upthe control circuit 110. For example, when considering an application toa wristwatch, in the condition in which the wristwatch is detached fromthe arm, the piezoelectric element 108 fails to generate electric power.Therefore, when the energy stored in the capacitor C has been workedout, the control circuit 110 becomes difficult to be restarted, and thepower generation unit 100 fails to start up the step-up operation.Therefore, it is necessary to once store the electrical power necessaryto restart the power generation unit 100.

Here, in the case in which the second switch SW2 and the first switchSW1 are both in the open state, the voltage generated by thepiezoelectric element 108 is full-wave rectified by the full bridgerectifier 120, and is then applied to the control circuit 110. Thepiezoelectric element 108 generates a voltage of about 2.5V althoughdepending on the amplitude of the vibration of the piezoelectric element108. The voltage is referred to as VC.

When performing the rectification using the full bridge rectifier 120,assuming that the forward voltage drop Vf is 0.4V, the voltage after therectification takes the value expressed by the following formula. Itshould be noted that in the case of using the full bridge rectifier 120,since the current flows through the diode two times, the voltagecorresponding to twofold of Vf is lost.

VC−2×Vf=2.5[V]−0.4[V]×2=1.7[V]

Since the voltage fails to reach the voltage (e.g., 3.3V) necessary tostart up the control circuit 110, the step-up operation fails to bestarted up with the voltage.

Therefore, by shorting the second switch SW2, and making the circuitoperate as the voltage doubler rectifier 120 a, the starting voltage ofthe control circuit is accumulated. The operation of accumulating thestarting voltage will be explained with reference to FIG. 4A. FIG. 4A isa circuit diagram showing a current path in the case of shorting thesecond switch SW2 according to the present embodiment. It should benoted that since the description that the piezoelectric element 108corresponds to the current source I0 and the capacitor C0 for storingthe charge is not so common as a drawing method of the voltage doublerrectifier diagram, the explanation will be continued using an equivalentcircuit of the piezoelectric element 108 having a voltage source V0 andthe capacitor C0 for storing the charge connected in series to eachother instead thereof. In the case in which the first electrode 109 aoutputs a negative voltage and the second electrode 109 b outputs apositive voltage, a current flows along the dotted arrow.

The current supplied from the voltage source V0 passes through thesecond electrode 109 b, then passes through the diode D2 and the firstelectrode 109 a, then charges the capacitor C0, and then returns to thevoltage source V0. In the case in which the first electrode 109 aoutputs a positive voltage and the second electrode 109 b outputs anegative voltage, a current flows along the solid arrow. In this case,the voltage of the capacitor C0 is added to the voltage of the voltagesource V0, and the capacitor C0 and the voltage source V0 function as asingle voltage source. The voltage of the capacitor C0 is obtained bysubtracting the voltage corresponding to the voltage drop in the diodeD2 from the voltage VC of the piezoelectric element 108. The currentfirstly passes through the capacitor C0, then passes through the firstelectrode 109 a, then passes through the diode D1, and then charges thecapacitor C. Then, the current passes through the second electrode 109b, and then returns to the voltage source V0. Here, since the voltagedrop Vf of the diode D1 is caused when charging the capacitor C, theinter-terminal voltage of the capacitor C takes the following value.

(VC−Cf)+(VC−Vf)=(2.5−0.4)+(2.5−0.4)=4.2[V]

FIG. 4B is an equivalent circuit diagram extracting the part related tothe path through which the current shown in FIG. 4A flows. Specifically,FIG. 4B is a circuit diagram extracting the components effectivelyfunctioning in the case of opening the first switch SW1 and shorting thesecond switch SW2. The circuit is a typical voltage doubler rectifier,and it becomes possible to supply a voltage roughly twofold of VC as thevoltage between the first electrode 109 a and the second electrode 109 bsandwiching the piezoelectric element 108 to thereby accumulate thestarting voltage of the control circuit.

Operation Sequence

Hereinafter, an operation sequence of the power generation unit 100described above will be explained. FIG. 5 is a flowchart of the presentembodiment for determining which one of the operation in the steadystate and the operation for obtaining a starting voltage is selected.

Firstly, as the step S1, whether or not the step-up operation can beperformed is determined. Specifically, whether or not the voltage higherthan the minimum starting voltage of the control circuit 110 is outputis determined.

If the voltage lower than the minimum starting voltage is output (N inthe step S1), the process proceeds to the step S5.

In the step S5, a step-up operation status is set to NG (the step-upoperation is disabled).

Then, as the step S6, the voltage doubler rectification is performed.

Then, as the step S4, whether or not the step-up operation can beperformed is determined.

If the voltage equal to or higher than the minimum starting voltage isoutput (Y in the step S4), the process returns to the step S2.

If the voltage lower than the minimum starting voltage is output (N inthe step S4), the process returns to the step S5.

If the voltage lower than the minimum starting voltage is output at thetime of START, the sequence described above is taken.

Further, if the voltage lower than the minimum starting voltage isoutput at the time of START, the sequence described above is taken.

If the voltage equal to or higher than the minimum starting voltage isoutput (Y in the step S1) in the step S1, the process proceeds to thestep S2.

In the step S2, the step-up operation status is set to OK (the step-upoperation is enabled).

Then, as the step S3, the step-up operation is performed.

Then, as the step S4, whether or not the step-up operation can beperformed is determined.

If the voltage equal to or higher than the minimum starting voltage isoutput (Y in the step S4), the process returns to the step S2.

If the voltage lower than the minimum starting voltage is output (N inthe step S4), the process returns to the step S5.

In this case, the operation sequence is performed with an infinite loop.If the operation sequence goes on, the operation fails to be stopped.Therefore, it is also preferable to provide a function of waiting for aBreak signal from the outside in the step S4, and then stopping theelectrical power generation if the Break signal is received (Breaksignal is present in the step S4). It should be noted that in the caseof, for example, generating the electrical power semipermanently, theinput process of the Break signal can be eliminated.

It should be noted that by changing the step-up operation status, itbecomes possible to perform control of, for example, deferring thestartup until the status is set to OK on a load, not shown, connected tothe power generation unit 100. The power generation unit described abovehas the following advantages.

As shown in FIGS. 3A through 3D, by using the resonant phenomenon in theresonant circuit provided with the inductor L and the capacitivecomponent C0 of the piezoelectric element 108, there can be obtained thevoltage with a larger amplitude than the voltage the piezoelectricelement 108 can output by itself as described above. Therefore, sincethe electrical power can be taken out from the piezoelectric element 108with a higher efficiency, it is possible to obtain a small-sized andefficient power generation unit.

As described in “Operation in Steady State,” the charge in thepiezoelectric element 108 increases every time the piezoelectric element108 is deformed, unless the charge is made to flow out of thepiezoelectric element 108. Therefore, the voltage between the terminalsof the piezoelectric element 108 increases. Therefore, if the losscaused when the charge flows through the inductor L and the first switchSW1, for example, is not considered, it is possible to graduallyincrease the voltage between the terminals of the piezoelectric element108. Therefore, it is possible to generate the electrical power at avoltage automatically raised to the voltage necessary to drive theelectrical load without providing an additional step-up circuit.

Once the voltage of the capacitor C drops to a level lower than thelower limit voltage with which the control circuit 110 can drive thefirst switch SW1, it becomes unachievable to actively control the firstswitch SW1, and it becomes unachievable to perform the electricalpower-generating operation described above. In this occasion, theoperation of switching the control circuit 110 from the full bridgerectifier 120 to the voltage doubler rectifier 120 a is performed.Therefore, the control circuit 110 is supplied with a voltageapproximately twofold of the voltage in the piezoelectric element 108.Since the voltage of the capacitor C exceeds the lower limit voltage,with which the control circuit 110 can drive the first switch SW1, byperforming the voltage doubler rectification, the control circuit 110 isswitched to the full bridge rectifier 120 and is operated in therectifying mechanism described above to thereby make it possible toprovide a power generation unit is possible to provide aself-recoverable and highly efficient power generation unit.

Since it becomes possible to switch the circuit from the full bridgerectifier 120 to the voltage doubler rectifier 120 a only by closing thesecond switch SW2, it becomes possible to provide a power generationunit provided with a high electrical power generation efficiency whilepreventing the number of components from increasing and capable ofachieving the self-recovery. In addition, since the full bridgerectifier 120 is obtained only by keeping the second switch SW2 open inthe case of keeping the voltage equal to or higher than the lower limitvoltage, it is possible to perform electrical power generation withoutdegrading the electrical power generation efficiency.

By using a normally-off switch as the first switch SW1 and a normally-onswitch as the second switch SW2, the full bridge rectifier 120 functionsas the voltage doubler rectifier 120 a if the voltage from the both endsof the capacitor C is stopped. Therefore, it is possible to provide apower generation unit capable of achieving the self-recovery and havinga high electrical power generation efficiency.

Secondary Cell

Hereinafter, an example of forming a secondary cell will be explained.FIG. 6 is a circuit diagram of the secondary cell according to thepresent embodiment. The secondary cell 200 is provided with a powergeneration unit 101, and a voltage regulator 130. Since the powergeneration unit 101 is described above, duplication of the explanationwill be avoided.

The voltage regulator 130 is supplied with the electrical power from thepower generation unit 101, and supplies a load not shown with theelectrical power. Although the example using the power generation unit101 is explained here, the power generation unit 100 can also be usedinstead thereof

The secondary cell 200 described above has the following advantages. Ifthere occurs the condition in which the inter-terminal voltage of thecapacitor C drops to a level lower than the lower limit voltagedescribed above, the voltage doubler rectification is performed asdescribed above once the vibration is applied. Then, when the voltageequal to or higher than the lower limit voltage is reached, theoperation is switched to the step-up operation, and it becomes possibleto efficiently provide the stabilized voltage to a load not shown underthe voltage control by the voltage regulator 130.

Electronic Apparatus

Hereinafter, an example of an electronic apparatus will be explained.FIG. 7 is schematic diagram showing a schematic structure of a pedometer(a manpo-kei, which is a registered trademark) as the electronicapparatus according to the present embodiment. The pedometer 1 isprovided with a reset button 10, a display section 11, and the secondarycell 200. In the case in which the pedometer 1 is at rest for a longperiod of time, the inter-terminal voltage of the capacitor C (see FIG.6) provided to the secondary cell 200 is held in the state of beinglower than the lower limit voltage described above.

Here, once a vibration is applied to the pedometer 1, the secondary cell200 performs the voltage doubler rectification as described above, thenperforms the step-up operation immediately after the voltage equal to orhigher than the lower limit voltage has been accumulated, and thus thepedometer 1 operates.

It should be noted that although the pedometer 1 is cited here as anexample of the electronic apparatus, the electronic apparatus is notlimited thereto, and the invention can also be applied to electronicapparatuses operating in response to a mechanical vibration, such as awristwatch or a wearable appliance. In particular, since the step-upoperation is a rectification method with a high efficiency, theinvention can preferably be applied to the electronic apparatuscorresponding to applications with high power consumption and requiredto operate wirelessly.

The electronic apparatus described above has the following advantages.The secondary cell 200 performs the step-up operation with a highelectrical power generation efficiency, and can therefore provide theelectrical power efficiently even with the vibration having a smallamplitude. Therefore, it becomes possible to add a calculation functionrequiring electrical power such as the calorie calculation to thefunction of the pedometer 1.

It should be noted that the invention is not limited to the embodimentsdescribed above, but various modifications or improvements can beprovided to the embodiments described above. Modified examples will bedescribed below. It should be noted that in the explanation of themodified examples, the constituents substantially the same as those inthe embodiments described above will be denoted by the same referencesymbols, and the explanation therefor will be omitted.

First Modified Example

FIG. 8 is a circuit diagram for explaining the present modified example.As shown in FIG. 8, a storage voltage detect circuit 150 for detectingthe inter-terminal voltage of the capacitor can also be disposed betweenthe both terminals of the capacitor C. The control circuit 110 performsthe step-up operation in the case in which the inter-terminal voltage ofthe capacitor C exceeds the lower limit voltage necessary for thestep-up operation, and performs the control of opening the first switchSW1 and shorting the second switch SW2 in the case in which theinter-terminal voltage is lower than the lower limit voltage. Thus, theswitching between the step-up operation and the voltage doublerrectification is performed efficiently, and it is possible to provide apower generation unit with a high electrical power generationefficiency.

Second Modified Example

FIGS. 9A and 9B are circuit diagrams for explaining the present modifiedexample. There is provided a charging state detection section 160 fordetermining whether or not a current is flowing from the piezoelectricelement 108 to the full bridge rectifier 120, and if the charging statedetection section 160 fails to detect charging during the step-upoperation, it is possible for the control circuit 110 to stop thestep-up operation, and at the same time, short the second switch SW2.The state in which the charging state detection section 160 fails todetect charging corresponds to the state in which no current is flowingfrom the piezoelectric element 108 to the capacitor C, and the state inwhich the step-up operation makes no contribution to the electricalpower generation. By stopping the step-up operation on this occasion, itis possible to prevent the electrical power for driving the first switchSW1 from being wasted.

Further, if the step-up operation is stopped, only the full bridgerectifier 120 is usually used. Therefore, the generation voltage of thepiezoelectric element 108 is dropped to a level lower than in the caseof performing the step-up operation, and it becomes difficult to makethe current flow from the piezoelectric element 108 to the capacitor C.However, by shorting the second switch SW2 to thereby switch the circuitto the voltage doubler rectifier 120 a, since the output voltage of thepiezoelectric element 108 increases to a level higher than in the fullbridge rectifier 120, it becomes easy to supply the capacitor C with thecurrent. According to the above configuration, a power generation unitwith a high electrical power generation efficiency can be provided.

It should be noted that it is also possible for the charging statedetection section 160 to detect the current actually flowing from thepiezoelectric element 108 to the full bridge rectifier 120. In thiscase, as shown in FIG. 9A, the charging state detection section 160 canbe substituted with the member 140 shown in FIG. 2A for detecting thetiming at which the deformation direction is switched. Besides theabove, there can also be cited a method in which the charging statedetection section 160 measures the voltages between the anode and thecathode of the diodes D1, D3 of the full bridge rectifier 120 to therebydetect whether or not the charging is in progress as shown in FIG. 9B.In this case, if the voltage on the anode side is higher than that onthe cathode side, it is determined that the current is flowing from thepiezoelectric element 108 to the electric storage element C, andtherefore, the capacitor C is in the charging state. This method doesnot require measurement of the faint current flowing from thepiezoelectric element, and is therefore easy to put into practice.

Third Modified Example

The explanation will be presented with reference to FIGS. 4A and 10.FIG. 10 is a circuit diagram for explaining the present modifiedexample. In the third and fourth modified examples, the full bridgerectifier 120 is formed using schottky barrier diodes. The schottkybarrier diode has a feature that the forward voltage drop is lower thanthat of the junction diode. Therefore, the loss corresponding to theforward voltage drop of the full bridge rectifier is reduced, the itbecomes possible to supply the capacitor with a higher voltage. On thecontrary, there is a disadvantage of a large reverse leakage current. Inthe case of operating as the voltage doubler rectifier 120 a describedabove, the diodes D3, D4 do not make a contribution to the rectification(it should be noted that the diode D4 is in the state in which the anodeand the cathode are shorted, and therefore has no tangible ill effect).The diode D3 making no contribution to the voltage doubler rectificationcauses the current to leak, and the electrical power generationefficiency to be degraded. On this occasion, by using the junction diodeonly as the diode D3, it is possible to prevent the leakage and to makethe voltage promptly reach the voltage with which the control circuit110 can operate. FIG. 10 shows a circuit diagram of the case in whichthe diode D3 is replaced with the junction diode.

Fourth Modified Example

The explanation will be presented with reference to FIGS. 4A and 11.FIG. 11 is a circuit diagram for explaining the present modifiedexample. In the case of operating as the voltage doubler rectifier 120 adescribed above, the diodes D3, D4 make no contribution to therectification (It should be noted that the diode D4 is in the state inwhich the anode and the cathode are shorted, and therefore has notangible ill effect). In other words, in the case of performing thevoltage doubler rectification, the circuit without the diode D3 issuperior, ideally. Therefore, by using, for example, a normally-off MOSswitch instead of the diode D3, it is possible to prevent the leakageand to make the voltage promptly reach the voltage with which thecontrol circuit 110 can operate. It should be noted that it is possibleto make the circuit perform the full-wave rectification whilesuppressing a loss in the forward direction by controlling the MOSswitch so as to perform the synchronous rectification after once thecontrol circuit 110 operates. FIG. 11 shows a circuit diagram of thecase in which the diode D3 is replaced with the switch SW3.

This application claims priority to Japanese Patent Application No.2011-218987, filed on Oct. 3, 2011, the entirety of which is herebyincorporated by reference.

What is claimed is:
 1. A power generation unit comprising: apiezoelectric element including a piezoelectric material; a deformingmember adapted to repeatedly deform the piezoelectric element; a pair ofelectrodes provided to the piezoelectric element; an inductorelectrically connected to the piezoelectric element; a first switchdisposed between the piezoelectric element and the inductor; a memberadapted to detect a timing at which a deformation direction of thedeforming member is switched; a full bridge rectifier adapted to rectifya current output from the pair of electrodes; a capacitor electricallyconnected to the full bridge rectifier, and adapted to store a currentsupplied from the full bridge rectifier; a second switch disposedbetween either one of the pair of electrodes and the capacitor; and acontrol circuit adapted to operate the first switch and the secondswitch.
 2. The power generation unit according to claim 1, wherein whena voltage stored in the capacitor reaches a voltage with which thecontrol circuit can drive the second switch, the control circuit setsthe first switch to an open state, and the second switch to a shortedstate.
 3. The power generation unit according to claim 1, wherein when avoltage charged in the capacitor reaches a voltage with which thecontrol circuit can drive the first switch and the second switch, thecontrol circuit sets the second switch to an open state, and performs acontrol of setting the first switch to a shorted state at the timing atwhich the deformation direction of the deforming member is switched, andthen setting the first switch to the open state after a timecorresponding to a half cycle of a resonance period of a resonantcircuit composed mainly of the inductor and the capacitor has elapsed.4. The power generation unit according to claim 1, further comprising: acharging state detection section adapted to detect a charging state ofthe capacitor, wherein when the charging state detection section detectsa state in which charging of the capacitor stops, the control circuitperforms control of setting the second switch to a shorted state, andsetting the first switch to an open state.
 5. A secondary cellcomprising: the power generation unit according to claim
 1. 6. Asecondary cell comprising: the power generation unit according to claim2.
 7. A secondary cell comprising: the power generation unit accordingto claim
 3. 8. A secondary cell comprising: the power generation unitaccording to claim
 4. 9. An electronic apparatus comprising: the powergeneration unit according to claim
 1. 10. An electronic apparatuscomprising: the power generation unit according to claim
 2. 11. Anelectronic apparatus comprising: the power generation unit according toclaim
 3. 12. An electronic apparatus comprising: the power generationunit according to claim 4.